Showing papers on "Atmospheric temperature range published in 1997"

Temperature-dependent oxygen electrochemistry on platinum low-index single crystal surfaces in acid solutions

Abstract: Using the rotating ring-disk technique (RRDPt(hkl)E), the oxygen-reduction reaction (ORR) was studied in sulfuric acid solution over the temperature range 298–333 K At the same temperature, the exc...

Investigation of the temperature dependence of dielectric relaxation in liquid water by thz reflection spectroscopy and molecular dynamics simulation

Abstract: We report measurements of the real and imaginary part of the dielectric constant of liquid water in the far-infrared region from 0.1 to 2.0 THz in a temperature range from 271.1 to 366.7 K. The data have been obtained with the use of THz time domain reflection spectroscopy, utilizing ultrashort electromagnetic pulses generated from a photoconductive antenna driven by femtosecond laser pulses. A Debye model with an additional relaxation time is used to fit the frequency dependence of the complex dielectric constants. We obtain a fast (fs) and a Debye (ps) relaxation time for the macroscopic polarization. The corresponding time correlation functions have been calculated with molecular dynamics simulations and are compared with experimental relaxation times. The temperature dependence of the Debye relaxation time is analyzed using three models: Transition state theory, a Debye–Stoke–Einstein relation between the viscosity and the Debye time, and a model stating that its temperature dependence can be extrapolated from a singularity of liquid water at 228 K. We find an excellent agreement between experiment and the two latter models. The simulations, however, present results with too large statistical error for establishing a relation for the temperature dependence.

Room-Temperature Spin Memory in Two-Dimensional Electron Gases

Abstract: Time-resolved Kerr reflectivity of two-dimensional electron gases in II-VI semiconductors provides a direct measure of electron spin precession and relaxation over a temperature range from 4 to 300 kelvin. The introduction of n-type dopants increases the electronic spin lifetimes several orders of magnitude relative to insulating counterparts, a trend that is also observed in doped bulk semiconductors. Because the electronic spin polarization in these systems survives for nanoseconds, far longer than the electron-hole recombination lifetime, this technique reveals thousands of spin precession cycles of 15 gigahertz per tesla within an electron gas. Remarkably, these spin beats are only weakly temperature dependent and persist to room temperature.

Real-time soil water dynamics using multisensor capacitance probes : Laboratory calibration

Abstract: There is a continued need for better methods to perform accurate, real-time, nearly continuous soil water measurements at specific depth intervals, with minimal soil disturbance, and covering field-scale areas. The objectives of this research were to assess the characteristics of a newly developed multisensor capacitance probe and to calibrate the sensors against a Mattapex silt loam (fine-silty, mixed, mesic Aquic Hapludult) soil. The soil was uniformly packed in small increments, with the aid of a large hydraulic press, in a wooden box (35.5 by 35.5 cm, 40.5 cm deep). Probe installation in the boxed soil mimicked that required for correct field installation. A highly significant (r2 = 0.992 for n = 15, and RMSE = 0.009 cm 3 cm -3 water), nonlinear (θ r = 0.490 SF 2.1674 ) relationship was found between the soil volumetric water content (0 v , cm 3 cm -3 ) and the scaled frequency |SF = (F a - F s )(F a -F w ) -1 ]. The SF represents the ratio of individual sensor's frequency (inside PVC pipe) response in soil (F,) compared with sensor responses in air (F,) and in nonsaline water (F w ) at room temperature ( 22°C). Axial and radial sensitivity studies showed that these capacitance sensors give integrated readings over a primary depth interval of 10 cm and a radial capacitance fringe within 10 cm of the wall of the access pipe. Temperature effects of air and water were measured, and the calculated effects on assessment of θ ν were less than the RMSE for a temperature range of 10 to 30°C. Our calibration studies indicate that these multisensor capacitance probes can be used to accurately measure volumetric soil water contents in a soil water monitoring system.

Thermal conductivity of germanium crystals with different isotopic compositions

Abstract: We have measured the thermal conductivity of seven germanium crystals with different isotopic compositions in the temperature range between 2 K and 300 K. These samples, including one made of highly enriched ${}^{70}\mathrm{Ge}(99.99%)$, show intrinsic behavior at room temperature with the exception of a $p$-type sample with $|{N}_{d}\ensuremath{-}{N}_{a}|\ensuremath{\cong}2\ifmmode\times\else\texttimes\fi{}{10}^{16} {\mathrm{cm}}^{\ensuremath{-}3}$. The ``undoped'' samples exhibit a ${T}^{3}$ dependence at low temperatures, basically determined by boundary scattering. The maximum value of $\ensuremath{\kappa}$ (which falls in the range between 13 K and 23 K) is found to be a monotonically decreasing function of the isotopic mass variance parameter $g$. The maximum ${\ensuremath{\kappa}}_{m}$ measured for the most highly enriched ${}^{70}\mathrm{Ge}(99.99%)$ sample is 10.5 kW/mK, one order of magnitude higher than for natural germanium. The experimental data have been fitted with the full Callaway theory, modified by treating transverse and longitudinal modes separately, using three free adjustable parameters for each set of modes to represent anharmonic effects plus the calculated contributions from isotopic and boundary scattering. For the isotopically purest ${}^{70}\mathrm{Ge}(99.99%)$ sample, dislocation scattering, or a similar mechanism, must be added in order to fit the data. We have also checked the effect of various surface treatments on the thermal conductivity in the low temperature region. The highest values of $\ensuremath{\kappa}$ are found after polish etching with a SYTON suspension.

Dependence of ultrasonic attenuation and absorption in dog soft tissues on temperature and thermal dose

Abstract: The effect of temperature and thermal dose (equivalent minutes at 43 °C) on ultrasonic attenuation in fresh dog muscle, liver, and kidney in vitro, was studied over a temperature range from room temperature to 70 °C. The effect of temperature on ultrasonic absorption in muscle was also studied. The attenuation experiments were performed at 4.32 MHz, and the absorption experiments at 4 MHz. Attenuation and absorption increased at temperatures higher than 50 °C, and eventually reached a maximum at 65 °C. The rate of change of tissue attenuation as a function of temperature was between 0.239 and 0.291 Np m−1 MHz−1 °C−1 over the temperature range 50–65 °C. A change in attenuation and absorption was observed at thermal doses of 100–1000 min, where a doubling of these loss coefficients was observed over that measured at 37 °C, presumably the result of changes in tissue composition. The maximum attenuation or absorption was reached at thermal dosages on the order of 107 min. It was found that the rate at which t...

Deep level defects in electron-irradiated 4H SiC epitaxial layers

Abstract: Deep level defects in electron-irradiated 4H SiC epitaxial layers grown by chemical vapor deposition were studied using deep level transient spectroscopy. The measurements performed on electron-irradiated p+n junctions in the temperature range 100–750 K revealed several electron traps and one hole trap with thermal ionization energies ranging from 0.35 to 1.65 eV. Most of these defects were already observed at a dose of irradiation as low as ≈5×1013 cm-2. Dose dependence and annealing behavior of the defects were investigated. For two of these electron traps, the electron capture cross section was measured. From the temperature dependence studies, the capture cross section of these two defects are shown to be temperature independent. © 1997 American Institute of Physics.

Oxidation of hydrogen on Ni/yttria-stabilized zirconia cermet anodes

Abstract: The oxidation of hydrogen on Ni/yttria-stabilized zirconia (Ni/YSZ) is studied by impedance spectroscopy. The active thickness obtained is 20 {micro}m or less. Conditions such as temperature, anodic overvoltage, electrode potential, H{sub 2} and H{sub 2}O partial pressure are varied. Three distinct arcs are identified in impedance spectra, representing at least three rate-limiting processes. One equivalent circuit of the type LR(RQ)(RQ)(RQ), where Q = Y{sub o}(j{omega}){sup n}, is used to describe all recorded impedance in the temperature range 850 to 1,000 C. The n-values are held constant, allowing a direct comparison of R and Y{sub o} values for different structures and conditions. The high-frequency arc (1 to 50 kHz) is sensitive to the cermet structure (particle size) and relatively insensitive to atmospheric composition and overvoltage. The related imperfect capacitance is suggested to be interpreted as a double-layer capacitance in the Ni/YSZ interface. The medium- (10 Hz to 1 kHz) and low-frequency arc (0.1 to 10 Hz) are sensitive to atmospheric composition and overvoltage. Both reaction resistances change their dependency on H{sub 2} partial pressure around 0.5 atm. The perfect capacitance related to the low-frequency arc is in the order of 0.5 to 2.5 F/cm{sup 2}, indicating an absorbed charged species rathermore » than surface adsorption.« less

Temperature-dependent thermal conductivity of porous silicon

Abstract: The thermal conductivity of electrochemically etched porous silicon (PS) layers was determined over a wide temperature range (T = 35 - 320 K) using the dynamic technique. Both the doping level of the silicon wafers (p and ) and the porosity P of the porous layers (P = 64 - 89%) were varied. The measured thermal conductivities were three to five orders of magnitude smaller than the values for bulk silicon. Furthermore, they increase with increasing the wafer doping level and with decreasing the porosity P of the layers. For all investigated PS layers the thermal conductivity increases with temperature. The results are discussed in terms of a simple model for heat conduction in PS based on the phonon diffusion model.

Studies on thermally stimulated shape memory effect of segmented polyurethanes

Abstract: The shape memory behavior of a series of polycaprolactone/diphenylmethane diisocyanate/butanediol (PCL/MDI/BDO) segmented polyurethanes of different composition was studied. The molecular weight of PCL diols was in the range of 1600-7000, and the hard segment content varied from 7.8 to 27% by weight. Film specimens for shape memory measurements were prepared by drawing at temperatures above the melting temperature of the soft segment crystals and subsequent quick cooling to room temperature under constrained conditions. The shape memory process was observed and recorded in a heating process. Parameters describing the recovery temperature, ability, and speed were used to study the influence of structure and processing conditions on the shape memory behavior of the sample. It was found that the high crystallinity of the soft segment regions at room temperature and the formation of stable hard segment domains acting as physical crosslinks in the temperature range above the melting temperature of the soft segment crystals are the two necessary conditions for a segmented copolymer with shape memory behavior. The response temperature of shape memory is dependent on the melting temperature of the soft segment crystals. The final recovery rate and the recovery speed are mainly related to the stability of the hard segment domains under stretching and are dependent on the hard segment content of the copolymers.

Does the low-temperature Arrhenius plot of the photoluminescence intensity in CdTe point towards an erroneous activation energy?

Abstract: Several experimental photoluminescence (PL) bands of different energies for variously prepared CdTe samples are compared. Temperature variation of the PL intensity is modeled with two nonradiative thermal activation energies, of which ET1 is dominant for about T⩽60 K, and ET2 for the upper temperature range of the measurement. The size of ET1 is invariably of the order of a few meV and, although of unclear origin, its magnitude is usually interpreted as an electronic energy level difference over which the carriers escape by thermal excitation. In CdTe the existence of such a small energy level difference ET1 is not easy to explain. On the contrary, we find clear evidence that, at low temperature, the PL intensity reduction with increasing temperature in fact results from the approximately T−2 temperature dependent capture cross sections of the carriers at the recombination centers, and not from a genuine thermal activation energy ET1.

Thermal Expansion of Periclase (MgO) and Tungsten (W) to Melting Temperatures

Abstract: In situ x-ray data on molar volumes of periclase and tungsten have been collected over the temperature range from 300 K to melting. We determine the temperature by combining the technique of spectroradiometry and electrical resistance wire heating. The thermal expansion (α) of periclase between 300 and 3100 K is given by α=2.6025 10−5+1.3535 10−8 T+6.5687 10−3 T−1−1.8281 T−2.

Adsorbent Helium Density Measurement and Its Effect on Adsorption Isotherms at High Pressure

Abstract: On the basis of an experimental study in a large temperature range, it is shown that “helium densities” of adsorbents measured at room temperature could be erroneous due to a non-negligible effect of helium adsorption. It is proposed that the density obtained with helium at high temperature, for instance, at the regeneration temperature of the adsorbent, be considered as the adsorbent density. Using the corrected densities of 3A, 4A, 5A, and 13X zeolites and of activated and graphitized carbons and of silica gel, we experimentally determined the adsorption of helium on the above mentioned adsorbents at room temperature and in a large pressure range up to 500 MPa. The shape of the adsorption isotherm reveals no saturation at high pressure. These experimental data are in agreement with Monte Carlo simulations of adsorption of a Lennard-Jones gas by a rigid plane as well as by a microporous rigid solid interface. We also examined implications of the new helium density of activated carbon for our previous mea...

Temperature dependence of yield stress, deformation mode and deformation structure in single crystals of TiAl (Ti−56 at.% Al)

Abstract: The plastic deformation behaviour of single crystals of TiAl with a composition of Ti-56 at.% Al has been studied in compression as a function of crystal orientation in the temperature range from −196 to 1100°C. The profile of yield stress-temperature curves for all orientations studied can be divided into three temperature regions; the yield stress rapidly decreases with increasing temperature at both low and high temperatures and an anomalous increase in yield stress is observed at intermediate temperatures, exhibiting a peak at 700–1000°C depending on crystal orientation. Both ordinary and [101] superlattice slip exhibit an anomalous increase in critical resolved shear stress (CRSS). However, the extent of the anomaly associated with the former slip is much smaller than that for the latter slip. The CRSS for [101] slip depends on crystal orientation. In the anomalous temperature region, the CRSS exhibits very small strain-rate sensitivity and thermal reversibility24Jun1996. Superlattice disloc...

Protein folding kinetics exhibit an Arrhenius temperature dependence when corrected for the temperature dependence of protein stability

Abstract: The anomalous temperature dependence of protein folding has received considerable attention. Here we show that the temperature dependence of the folding of protein L becomes extremely simple when the effects of temperature on protein stability are corrected for; the logarithm of the folding rate is a linear function of 1/T on constant stability contours in the temperature–denaturant plane. This convincingly demonstrates that the anomalous temperature dependence of folding derives from the temperature dependence of the interactions that stabilize proteins, rather than from the super Arrhenius temperature dependence predicted for the configurational diffusion constant on a rough energy landscape. However, because of the limited temperature range accessible to experiment, the results do not rule out models with higher order temperature dependences. The significance of the slope of the stability-corrected Arrhenius plots is discussed.

Liquid water in the domain of cubic crystalline ice Ic.

Abstract: Vapor-deposited amorphous water ice when warmed above the glass transition temperature (120-140 K), is a viscous liquid which exhibits a viscosity vs temperature relationship different from that of liquid water at room temperature. New studies of thin water ice films now demonstrate that viscous liquid water persists in the temperature range 140-210 K. where it coexists with cubic crystalline ice. The liquid character of amorphous water above the glass transition is demonstrated by (1) changes in the morphology of water ice films on a nonwetting surface observed in transmission electron microscopy (TEM) at around 175 K during slow warming, (2) changes in the binding energy of water molecules measured in temperature programmed desorption (TPD) studies, and (3) changes in the shape of the 3.07 micrometers absorption band observed in grazing angle reflection-absorption infrared spectroscopy (RAIRS) during annealing at high temperature. whereby the decreased roughness of the water surface is thought to cause changes in the selection rules for the excitation of O-H stretch vibrations. Because it is present over such a wide range of temperatures, we propose that this form of liquid water is a common material in nature. where it is expected to exist in the subsurface layers of comets and on the surfaces of some planets and satellites.

Phase equilibrium (solid-liquid-gas) in polyethyleneglycol-carbon dioxide systems

Abstract: Phase equilibria (solid-liquid-gas) for polyethyleneglycol (with molar masses 1500, 4000, 8000 and 35 000 g mol−1) and carbon dioxide mixtures were determined. It was found that the S-L-V lines in P-T projection for all polyethyleneglycols studied show a temperature minimum and a temperature maximum. These phenomena have not been reported previously in the literature. If a reduced temperature (obtained by dividing the temperature of the solid-liquid transition under pressure by the melting point of PEG at 1 bar) is introduced, the S-L transition of all binary systems are represented by one line. The solubility of CO2 in liquid polyethyleneglycols is influenced by pressure and temperature, and is almost independent of the molecular mass of polyethyleneglycol. In a P-T projection, the vapour-liquid equilibrium is represented by a straight line if isopleths are plotted as a function of the reduced temperature. By combining the P-Tred projections of the S-L and L-V transitions, one master diagram can obtained which is valid for all binary mixtures of polyethyleneglycol and CO2 in the pressure and temperature range investigated.

Pressure and Temperature Dependence of the Gas-Phase Reaction of SO3 with H2O and the Heterogeneous Reaction of SO3 with H2O/H2SO4 Surfaces

Abstract: The gas-phase reaction of SO3 with H2O and the heterogeneous reaction of SO3 with H2O−H2SO4 surfaces have been studied in a fast flow reactor coupled to a chemical ionization mass spectrometer (CIMS) for species detection. The gas-phase reaction was studied under turbulent flow conditions over the pressure range from 100 to 760 Torr N2 and the temperature range from 283 to 370 K. The loss rate of SO3 was measured under pseudo-first-order conditions; it exhibits a second-order dependence on water vapor concentration and has a strong negative temperature dependence. The first-order rate coefficient for the SO3 loss by gas-phase reaction shows no significant pressure dependence and can be expressed as kI(s-1) = 3.90 × 10-41 exp(6830.6/T)[H2O]2 where [H2O] is in units of molecule cm-3 and T is in Kelvin. The overall uncertainty of our experimentally determined rate coefficients is estimated to be ±20%. At sufficiently low SO3 concentrations (<1012 molecule cm-3) the rate coefficient is independent of the init...

Chain Mobility in Polymer Systems: on the Borderline between Solid and Melt. 1. Lamellar Doubling during Annealing of Polyethylene

Abstract: In-situ small-angle X-ray scattering (SAXS) and low-frequency Raman spectroscopy experiments have been performed to study the mobility and motion of polymer chains upon annealing in a temperature range close to but below the melting temperature. Ultrahigh molecular weight polyethylene, UHMW-PE, was taken as the model polymer, which was crystallized from solution. Regularly stacked lamellar crystals could be obtained after drying the solution-crystallized films. SAXS studies revealed that, upon heating above 110 °C, the lamellar thickness (long period) increases to twice the initial value. This quantum increase was confirmed by transmission electron microscopy (TEM). By means of in-situ low-frequency Raman spectroscopy it was observed that during heating a shift occurs in the longitudinal acoustic (LA) Raman frequency to lower values, indicative of an increase in the thickness of the crystalline core. The combined experimental observations indicate that thickening occurs via a mutual chain rearrangement be...

Dielectric behavior of lead magnesium niobate relaxors

Abstract: The dielectric behavior of a solid solution, 10 mol % lead titanate in lead magnesium niobate, is measured at different frequencies from 100 Hz to 100 kHz in the temperature range from -100 to 120 \ifmmode^\circ\else\textdegree\fi{}C. A standardizing method is introduced to analyze the curve of the dielectric constant vs temperature. It results a master curve behavior between the dielectric constant and temperature at temperatures higher than the temperature of the dielectric constant maximum. The dielectric relaxation behavior is analyzed with various models. The best way to characterize the degree of the dielectric relaxation for relaxor ferroelectrics is established using the experimental data. It is indicated that the temperature dependence of the static dielectric constant can be well described by an exponential function, while the temperature dependence of the relaxation time is described by a superexponential function. Based on the specialty of the relaxor ferroelectrics, a distribution function for the relaxation times is introduced and a model is introduced to simulate the dielectric behavior of the relaxor ferroelectrics. The model can express well both the temperature and frequency dependence of the dielectric behavior for a relaxor ferroelectrics. All of the parameters in the fitting formula can be experimentally determined. The model shows that in the low-temperature range, there are two simple relationships about the dielectric frequency spectrum: \ensuremath{\varepsilon}\ensuremath{''}(\ensuremath{\omega},T)=(-\ensuremath{\pi}/2)\ensuremath{\partial}\ensuremath{\varepsilon}\ensuremath{'}(\ensuremath{\omega},T)/\ensuremath{\partial} ln \ensuremath{\omega} and \ensuremath{\varepsilon}\ensuremath{'}=B(T)(ln ${\mathrm{\ensuremath{\omega}}}_{0}$-ln \ensuremath{\omega}). These relationships are verified by the experimental results. A way to obtain the accurate value of ${\mathrm{\ensuremath{\varepsilon}}}_{\mathrm{\ensuremath{\infty}}}$ in the low-temperature range is described.

Atomistic studies of O2 dissociation on Pt(111) induced by photons, electrons, and by heating

Abstract: The adsorption and subsequent dissociation of O2 on Pt(111) was studied by variable temperature scanning tunneling microscopy in the temperature range of 40 to 215 K. Tight clustering of bridge site molecules is observed on terraces between 40 and 70 K, indicating a highly mobile precursor to chemisorption. Coexistence of bridge and fcc hollow site molecules in fractal-shaped islands is observed after dosing between 70 and 95 K. Dissociation of these species was induced by uv radiation, inelastic tunneling electrons, and heating. In all three cases, two O atoms are found within two lattice constants of the original molecule and one to three lattice constants apart.

Thermal Property, Structure, and Dynamics of Supercooled Water in Porous Silica by Calorimetry, Neutron Scattering, and NMR Relaxation

Abstract: Thermal properties, structure, and dynamics of supercooled water in porous silica of two different pore sizes (30 and 100 A) have been investigated over a temperature range from 298 down to 193 K by differential scanning calorimetry (DSC), neutron diffraction, neutron quasi-elastic scattering, and proton NMR relaxation methods. Cooling curves by DSC showed that water in the 30 A pores freezes at around 237 K, whereas water in the 100 A pores does at 252 K. Neutron diffraction data for water in the 30 A pores revealed that with lowering temperatures below 237 K hydrogen bond networks are gradually strengthened, the structure correlation being extended to 10 A at 193 K. It has also been found that crystalline ice is not formed in the 30 A pores in the temperature range investigated, whereas cubic ice (Ic) crystallizes in the 100 A pores at 238 K. The self-diffusion coefficients of water protons in both pores determined from the quasi-elastic neutron scattering measurements showed that the translational moti...